1. Field of Endeavor
The present invention relates to optics and more particularly to a system that mitigates the growth of damage in an optic.
2. State of Technology
U.S. Pat. No. 4,667,101 for predicting threshold and location of laser damage on optical surfaces by Wigbert Siekhaus, patented May 19, 1987 provides the following description, xe2x80x9cModern day applications of laser devices call for increasingly powerful and precise beams. Such applications require high resolution optical devices such as lenses, filters, and mirrors. The application of large intensities of laser energy to these devices frequently destroys them during operation. Often the level of intensity required for experimental applications (such as the Projects Nova and Novette at the Lawrence Livermore National Laboratory) is so high that pretesting of the optical device at the required intensities would be impractical. The level of effort required to prepare for and execute the desired experiments, however, is very high and so an effective means of pretesting such devices is desirable. Presently there are no commercially available devices capable of xe2x80x9cstress testingxe2x80x9d a particular optical device. U.S. Pat. No. 3,999,865, issued Dec. 28, 1976 to Milam et al., teaches an instrument capable of analyzing the cause of damage to optical devices. It provides for subjecting the device to a damaging energy and intensity and then analyzing the damage from the standpoint of time and applied power in order to determine the one or more of several reasons for the laser induced damage. While Milam is helpful in improving system design or production techniques, it requires that damage actually occur and only indirectly identifies flaws through analysis of the parameters of the damaging event. The tested device clearly can no longer be used.xe2x80x9d
U.S. Pat. No. 5,143,533 for a method of producing amorphous thin films by Raymond M. Brusasco, patented Sep. 1, 1992 provides the following description, xe2x80x9cDisclosed is a method of producing thin films by sintering which comprises: a. coating a substrate with a thin film of an inorganic glass forming parulate material possessing the capability of being sintered, and b. irridiating said thin film of said particulate material with a laser beam of sufficient power to cause sintering of said material below the temperature of liquidus thereof. Also disclosed is the article produced by the method claimed.xe2x80x9d
U.S. Pat. No. 5,472,748 for permanent laser conditioning of thin film optical materials by Wolfe et al., patented Dec. 5, 1995, provides the following description: xe2x80x9cThe performance of high peak power lasers, such as those used for fusion research and materials processing, is often limited by the damage threshold of optical components that comprise the laser chain. In particular, optical thin films generally have lower damage thresholds than bulk optical materials, and therefore thin films limit the output performance of these laser systems. Optical thin films are used as high reflectors, polarizers, beam splitters and anti-reflection coatings. The Nova project at Lawrence Livermore National Laboratory is designed to study the use of lasers to produce fusion by inertial confinement. The 1.06 xcexcm wavelength Nova laser output is limited, in part, by the damage threshold of large aperture (approximately 1 m diameter) dielectric thin films coated on flat substrates. Proposed future fusion lasers require optical coatings with laser induced damage thresholds that exceed a fluence of 35 J/cm 2 in 10 ns pulses at the 1.06 xcexcm wavelength. Fluence is defined in the specification and claims for a pulsed laser of a specified wavelength and specified pulse length as the energy per unit area delivered by a single pulse. Prior to the invention, the highest damage thresholds were in the range from 10-20 J/cm2 in a 10 ns pulse at the 1.06 xcexcm wavelength. Therefore, a method of increasing the laser damage threshold of dielectric optical thin films (or coatings) is needed.xe2x80x9d
U.S. Pat. No. 5,796,523 for a laser damage control for optical assembly by John M. Hall, patented Aug. 18, 1998 provides the following description, xe2x80x9cProtection methods and apparatus for optical equipment have been attempted for providing protection from laser energy that could otherwise damage optical radiation detectors, including the human eye. The most common technique of providing protection involves optical filtering elements, which offer substantial protection but only over a limited, fixed spectral color range. Standard dielectric coatings are the most common form of filters, and flat plates with these xe2x80x9cnotchxe2x80x9d coatings can be easily inserted into or outside many common optical assemblies. As noted above, however, these filters are useful only over a limited range of wavelengths, and also have the added disadvantage of blocking even non-harmful radiation within the designed spectral region. Typical military magnifying optical assemblies such as telescopes, periscopes, and binoculars vary widely, and typically have magnifying powers ranging from 4xc3x97to 10xc3x97, with entrance aperture diameters going from 20 mm to 60 mm or more. As the magnifying power increases, the angular resolution increases, and thus the farther away a given target can be recognized. The larger apertures are required to gather sufficient light energy to allow good contrast for far-away targets. These magnifying optical systems are commonly designed for use with the human eye, but can also easily perform similar tasks when connected to standard television camera equipment. Given the harsh nature of military environments, these optical systems do not lend themselves easily to the use of attachments to perform laser protection functions. All magnifying optical assemblies of the kind found in telescopes, periscopes, and binoculars can be characterized as consisting of an objective lens set, followed by an eyepiece assembly, with either a real or virtual focal plane between, as well as a variety of intervening prism assemblies (almost always porro prisms) to keep the image orientation proper. The magnifying power is defined as the ratio of the objective focal length divided by the eyepiece focal length. Typical fields of view for these systems range from 2xc2x0 to 10xc2x0, depending upon the magnification. In the prior art for all these systems, the focal planes between the objective and eyepiece sections, or between any intervening relay optics, is not well corrected for aberrations. This does not affect the overall system performance, because the aberrations of the objective can be compensated by those of the eyepiece. It is much more difficult to design both objective and eyepiece optics to each have diffraction limited focal planes, and therefore this feature is not normally embraced by the current art. Additionally, since the magnifying power is the ratio of the objective and eyepiece focal lengths, it is desirable to have a relatively short focal length eyepiece to minimize the objective focal length for a given magnification. This reduces the overall size of the system, but does not offer much room between the eyepiece assembly and the intermediate focal plane. Because of this, prior art designs do not usually allow elements other than thin transmissive reticle plates to occupy the space in or near the intermediate focal plane. The prior art in developing laser protective devices offers many techniques, including sacrificial mirrors, transmissive optical power limiters, liquid cells, etc. These devices are generally designed to operate passively within an optical system until indicent optical radiation is of sufficiently high energy to activate the protective mechanism. In order to set the activation threshold below the damage threshold of the detector (human eye, TV camera, etc.), it is desirable to place the power limiter in or near a well corrected, diffraction limited focal plane. Additionally, the optical system must be able to accommodate the volume of the power limiter device, and be able to provide proper image orientation should the device create an image translation.xe2x80x9d
U.S. Pat. No. 6,099,389 for fabrication of an optical component by Nichols et al., patented Aug. 8, 2000, provides the following description: xe2x80x9cA method for forming optical parts used in laser optical systems such as high energy lasers, high average power lasers, semiconductor capital equipment and medical devices. The optical parts will not damage during the operation of high power lasers in the ultra-violet light range. A blank is first ground using a fixed abrasive grinding method to remove the subsurface damage formed during the fabrication of the blank. The next step grinds and polishes the edges and forms bevels to reduce the amount of fused-glass contaminants in the subsequent steps. A loose abrasive grind removes the subsurface damage formed during the fixed abrasive or xe2x80x9cblanchardxe2x80x9d removal process. After repolishing the bevels and performing an optional fluoride etch, the surface of the blank is polished using a zirconia slurry. Any subsurface damage formed during the loose abrasive grind will be removed during this zirconia polish. A post polish etch may be performed to remove any redeposited contaminants. Another method uses a ceria polishing step to remove the subsurface damage formed during the loose abrasive grind. However, any residual ceria may interfere with the optical properties of the finished part. Therefore, the ceria and other contaminants are removed by performing either a zirconia polish after the ceria polish or a post ceria polish etch.xe2x80x9d
International Patent Application Number WO 01/54853 for a method and apparatus for repair of defects in materials with short laser pulses by Paul, B. Corkum et al., published Aug. 2, 2001, provides the following information, xe2x80x9cTypically, a method of laser repair requires two key stepsxe2x80x94 locating defects precisely and controlling the laser beam to impact only on the places where defects are detected. Simple though these requirements appear, it is often difficult to achieve both. For example in repairing some electronic or optoelectronic devices, some defects and/or the effect of the defects are not easily identified until the devices are activated. Since laser beam repair devices often operate on wafers to correct identified problems, activating individual devices is not a trivial task.xe2x80x9d
Features and advantages of the present invention will become apparent from the following description. Applicants are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the invention. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention. The scope of the invention is not intended to be limited to the particular forms disclosed and the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.
The present invention relates to a system of reducing or eliminating the growth of laser-induced damage sites in optics upon continued irradiation with high peak power lasers with wavelengths ranging from the infrared to the ultraviolet. The invention also relates to a method of reducing or eliminating the density of laser-induced damage initiation sites in optics upon continued irradiation with high peak power lasers with wavelengths ranging from the infrared to the ultraviolet.
The present invention provides a glass optic for operation at the fundamental Nd:YAG laser wavelength of 1.06 micrometers through and including the tripled Nd:YAG laser wavelength of 355 nanometers produced by the method of reducing or eliminating the growth of laser damage sites in the optics by processing the optics to stop damage in the optics from growing to a predetermined critical size. One embodiment is produced by etching the damage sites with an electrical discharge plasma. Another embodiment is produced by local removal of glass and absorbing material via electrical discharge plasma etching. Another embodiment is produced by removing local surface height variations in the area and rendering the area more smooth by use of electrical discharge plasma etching. Another embodiment is produced by elimination of cracks emanating from the initial laser damage sites performed by chemical etching of the glass material matrix by electrical discharge plasma etching. Another embodiment is produced by imposing compressive stress into the uppermost layer of the optics and rendering flaws within the compressive stress layer in such a state that they may not grow. Another embodiment is produced by local removal of glass and absorbing material via carbon dioxide laser irradiation. Another embodiment is produced by depositing silica on the damage sites with an electrical discharge plasma. Another embodiment is produced by illuminating the damage sites using a small aperture CO2 laser beam. In another embodiment the beam delivers up to 200 watts of power in a beam up to 10 mm in diameter. In another embodiment the beam is focused into an area between 20 and 1500 microns in diameter. In another embodiment the beam has a beam spatial profile that is smooth Gaussian. In another embodiment the beam possess a beam power stability of xe2x89xa61%.
An embodiment of the present invention is useful in that the embodiment permanently reduces or eliminates the growth of laser-induced damage sites in optics upon continued irradiation at the laser wavelength of 1.06 micron through and including the tripled wavelength of 0.35 micron.
Another embodiment of the present invention is useful in that the embodiment reduces the density of damage initiators in glass optics by preinitiating damage on the said optics with a 0.35-micron laser and then reducing or eliminating the growth of the preinitiated damage site on the said optics.
Another embodiment of the present invention is useful in that the embodiment provides a method and apparatus for reducing or eliminating the growth of laser-induced damage sites in large-aperture glass optics upon continued irradiation at the laser wavelength of 1.06 micron through and including the tripled wavelength of 0.35 micron.
Another embodiment of the present invention is useful in that the embodiment provides a process of mitigating laser-induced damage growth in glass optics by virtue of very localized removal of glass and absorbing material. The invention is also a process of mitigating laser-induced damage growth in glass optics by virtue of modification of the local structure of the glass in the affected area. Another embodiment of the present invention is useful in that the embodiment provides a process of mitigating laser-induced damage growth in glass optics by virtue of elimination of cracks emanating from the initial damage site. Another embodiment of the present invention is useful in that the embodiment provides a process of mitigating laser-induced damage growth in glass optics by virtue of removing local height variations in the affected area and rendering the affected area smoother.
Other embodiments of the present invention are useful in that the embodiments provide at least three methods for local treatment of the damage site:
Etching the damage site with an electrical discharge plasma.
Depositing silica on the damage site with an electrical discharge plasma.
Illuminating the damage site using a small aperture CO2 laser beam.
The present invention provides method of reducing or eliminating the growth of laser damage sites in optics for optics that are expected to subsequently be operated at the fundamental Nd:YAG laser wavelength of 1.06 micrometers through and including the tripled Nd:YAG laser wavelength of 355 nanometers. The method includes the steps of processing the optics to stop damage in the optics from growing to a predetermined critical size. In one embodiment the damage sites are etched with an electrical discharge plasma. Another includes local removal of glass and absorbing material via electrical discharge plasma etching. Another embodiment includes mitigating the growth of laser damage sites in optics by virtue of modification of the local structure of the glass in the affected area, removing local surface height variations in the area, and rendering the area more smooth by use of electrical discharge plasma etching. Another embodiment includes mitigating the growth of laser damage sites in optics by virtue of elimination of cracks emanating from the initial laser damage sites performed by chemical etching of the glass material matrix by electrical discharge plasma etching. Another embodiment includes mitigating the growth of laser damage sites in optics by virtue of modification of the local distributions of stress in the glass matrix by imposing compressive stress into the uppermost layer of the optics and rendering flaws within the compressive stress layer in such a state that they may not grow. Another embodiment includes local removal of glass and absorbing material via carbon dioxide laser irradiation. Another embodiment includes mitigating damage in optics by virtue of elimination of cracks emanating from the initial laser damage sites performed by irradiation of the local area with a carbon dioxide laser. Another embodiment includes depositing silica on the damage sites with an electrical discharge plasma. Another embodiment includes illuminating the damage sites using a small aperture CO2 laser beam. Another embodiment includes delivering up to 200 watts of power in a beam up to 10 mm in diameter. Another embodiment includes focusing the beam into an area between 20 and 1500 microns in diameter. In another embodiment the beam has a beam spatial profile that is smooth Gaussian. In another embodiment the beam has a beam possess a beam power stability of xe2x89xa61%. In another embodiment the focus and power level of the CO2 laser is adjusted to produce treatment pits as shallow as tenths of a micron and diameters of less than a hundred microns. In another embodiment the damage sites are etched with an electrical discharge plasma and operating at the fundamental Nd:YAG laser wavelength of 1.06 micrometers through and including the tripled Nd:YAG laser wavelength of 355 nanometers.
In one embodiment a laser method is provided including the steps of irradiation of the local area with a carbon dioxide laser and operating at the fundamental Nd:YAG laser wavelength of 1.06 micrometers through and including the tripled Nd:YAG laser wavelength of 355 nanometers. Another embodiment includes scanning the optics to initiate small-scale damage sites and removing nanoscale impurities. The impurities are expelled by production of the damage sites with photon absorbing components and curing the damaged sites by smoothing out the damage sites photon absorbing components, annealing any cracks, and eliminating any absorbing species that could cause damage growth.
Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description and by practice of the invention.